1. Field of the Invention
The present invention relates to an image processing method used in printing an image on a print medium by using a connecting head in which a plurality of chips, each of which is provided with a nozzle array for ejecting ink of the same color, are arranged.
2. Description of the Related Art
There is conventionally provided a full line type inkjet printer in which, relative to a print head in which nozzles are arranged as wide as to cover a region corresponding to a width of a print medium, the print medium is conveyed in a direction intersecting with the arrangement direction of the nozzles, thus printing an image on the print medium. This full line type inkjet printer has recently attracted attention because of capability of the high speed output. This full line type inkjet printer adopts an elongated print head in which a great number of nozzles are arranged. However, when many nozzles are manufactured together for such a print head, a yield rate on the manufacturing process is degraded. Therefore, a recent full line type inkjet printer is generally manufactured such that a plurality of the same type chips, each of which has a predetermined number of nozzles arranged therein, are prepared and are arranged in such a manner that the nozzles are located successively.
FIG. 1A and FIG. 1B are diagrams each explaining a construction example of a connecting head disclosed in Japanese Patent Laid-Open No. H05-057965(1993). In FIG. 1A, a plurality of chips 202, in each of which a plurality of nozzles 201 are formed, are arranged to extend in y direction equal to a nozzle arrangement direction of each chip. At this time, neighboring chips each other are arranged in y direction by being alternately shifted in x direction in such a manner that an arrangement pitch of nozzles in y direction is constant also in a connecting portion (overlap region) between one of the chips and the other. The connecting head is generally configured in such a manner that a plurality of chips are arranged successively in y direction to thus form a predetermined overlap region of the chips. Ink is ejected in a constant frequency from the plurality of the nozzles and at the same time the print medium is conveyed in x direction at a speed corresponding to such frequency, thus printing an image on the print medium.
FIG. 1B is a diagram showing an example in which four nozzle arrays are formed on one chip. In the construction where a plurality of nozzle arrays are thus formed on one chip, the same line extending in the conveyance direction (x direction) on the print medium can be alternately printed by a plurality of nozzles. Therefore, even if a nozzle having an ejection defect such as a non-ejection or a shift of an ejection direction is included in the chip, the dot printed by such a defect nozzle does not exist successively in x direction and therefore an uniform image with a scattering ejection characteristic can be outputted. In the recent full line type inkjet printer, a plurality of nozzle arrays is generally formed on one chip as shown in FIG. 1B.
Incidentally in the overlap region of the aforementioned connecting head, the number of nozzles printing the same line extending in x direction is more than that in the non-connecting region. For example, in a case of FIG. 1A, the number of the nozzle printing the same line is one in the non-overlap region and the number of the nozzle printing the same line is two in the overlap region. In a case of FIG. 1B, the number of the nozzle printing the same line is four in the non-overlap region and the number of the nozzle printing the same line is eight in the overlap region. In Japanese Patent Laid-Open No. H05-057965(1993), continuity of dots in x direction is maintained or surface irregularity due to variations of the ejection characteristic in y direction is restricted by alternately printing dots with all the nozzles thus included in the overlap region. Particularly since a so-called gradation mask for gradually changing a printing rate (print allowance rate) between plural chips in the overlap region corresponding to a nozzle position is used in Japanese Patent Laid-Open No. H05-057965(1993), the continuity of the dots in y direction is more smoothed.
However, in the printing in the overlap region, a location region of the nozzles used for the printing is longer in x direction as compared to that in the printing in the non-overlap region. Therefore, it is confirmed that in a case where the print medium conveyed in x direction is slightly inclined, the influence due to the inclination tends to easily occur in an image.
FIG. 2A and FIG. 2B are diagrams explaining a state between a conveyance direction of the print medium and an image adverse effect in the overlap region of chips. For simplification herein, there is shown an example of a print head 500 where a chip 501 and a chip 502 are arranged in such a manner that each chip has two nozzle arrays formed thereon to have an overlap region corresponding to four nozzles of each nozzle array in y direction.
By referring to FIG. 2A, the same line extending in x direction is printed by four nozzles as a sum of two nozzles in the chip 501 and two nozzles in the chip 502 in a region included in the overlap region. When the print medium is accurately conveyed in x direction to the print head 500, dots to be printed by nozzles 503A, 503B, 504A and 504B are printed on the same line 505 extending in x direction. That is, if ink is ejected toward the same position on the print medium by the nozzles 503B and 504A, two overlapped dots as denoted at 506 are formed on the print medium.
On the other hand, FIG. 2B is a case where the print medium is conveyed on an inclination of θ to x direction. In this case, dots printed by the nozzles 503A, 503B, 504A and 504B are not printed on the same line 505 extending in x direction. That is, if ink is ejected toward the same position on the print medium by the nozzles 503B and 504A, two separate dots as shown by a dot 508 and a dot 509 are formed on the print medium. At this time, a distance c between the two separate dots can be, in a case where a distance between the nozzle 503B and the nozzle 504A in x direction is defined as a, expressed by a formula of c a×tan θ.
There are some cases where such a shift in the print position between nozzle arrays in an overlap region changes a coverage of the dot on the print medium to give an influence on an image density or makes graininess of the dot easily stand out to degrade an image itself. In addition, as a difference in density or graininess between the overlap region and the non-overlap region can be confirmed, this difference is recognized as unevenness in the image.